Solvents used as mobile phase in a liquid chromatograph are in their original state usually saturated with air under atmospheric pressure. This may lead to several faults during operation of the chromatograph, e.g., air bubbles may be generated in the region of the suction valves of the solvent pump, causing flow instabilities. Particularly, this occurs when generating solvent gradients, i.e., time dependent controlled variation of the composition ratio of two or more solvents. In order to generate such gradients, two or more solvents, such as water and methanol are mixed before entering the suction valve of the pump. Often the amount of air maximally soluble in the mixture is lower than the sum of the amounts of air maximally soluble in the single mixture components. This causes degassing during the mixing procedure. Additionally, the air dissolved in the solvents can cause errors in the output signals of the sample detectors, either by generating air bubbles in the small volume detector cells, or by affecting the physical or chemical effect on which the detection is based. The above-described problems are discussed in "CZ-Chemietechnik" 1(1972) pgs. 73-78, as well as in A. Zlatkis (editor): "Advances in Chromatography", Houston, Texas, 1978, pages 277-293.
In order to avoid these problems several techniques for solvent degassing have already been proposed and practically used. For example, from Hewlett-Packard Technical Information Bulletin April, 1973, entitled "High-Speed Liquid Chromatograph Model 1010A" it is known to heat and simultaneously to stir the solvent in a closed vessel which is evacuated by a vacuum pump connected to it. However, since most solvents are inflammable, sufficiently safe heating devices require an expensive design.
Another prior art method is an ultrasonics treatment of the solvent, such as described in "Analytical Chemistry", Volume 46, No. 9, August 1974, pgs. 1365-1366. However, this is still more expensive than the above-described solvent heating technique. Moreover, comparison of measurements has shown that it is less effective.
Yet another known method is to introduce a helium stream into the solvent (see e.g. Spectra-Physics Product Bulletin B005 April, 1979, entitled "AP8000 High-Performance Liquid Chromatograph"). By this treatment the dissolved air is "expelled" and instead of the air, helium is dissolved in the solvent. This is per se an effective method requiring only simple devices. However, since the saturation of the solvent with dissolved helium occurs under atmospheric pressure and the saturation solubility is proportional to the static pressure within the solvent, gas bubble generation may occur in system regions on the suction side of the pump, such as in valves or tubes having lower than atmospheric pressure.